Publication Date:
2014-01-03
Description:
Chromium K -edge X-ray absorption spectra were collected to characterize Cr in forsterite (Mg 2 SiO 4 ) as well as sulfides within the MAC 88136 EL3 chondrite to determine Cr valence and to see whether forsterite within this meteorite can be used as a Cr 2+ -silicate standard. Spectra were measured on several areas within a nearly pure 100 x 200 μm forsterite grain containing 0.13 wt% Cr. XANES findings indicate highly reduced Cr 2+ species, with no clear evidence of Cr 3+ or Cr 6+ . EXAFS data indicate an average 2.02 Å Cr-O nearest-neighbor distance, consistent with Cr-O distances found in square-planar Cr 2+ O 4 sites observed in synthetic crystalline silicates, and an average 2.69 Å Cr-Si second-nearest neighbor distance, consistent with Cr 2+ substituting for Mg 2+ in the forsterite M(1) site. Nearest-neighbor Debye-Waller factor and coordination number parameters indicate Cr 2+ is likely entering forsterite in disordered sites that are possible intermediates between M(1) and square-planar Cr 2+ O 4 configurations. Preliminary Cr XAS measurements on sulfides within this meteorite also indicate Cr 2+ in CrS 6 octahedra.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
